Mammary tumorigenesis in mice is influenced by chemical carcinogens, hormones and the genetic complement. Among the factors in the genetic complement that appear to influence mammary tumorigenesis is the endogenous, germinally-transmitted murine mammary tumor virus (MMTV). This proposal will examine the relationship between germinally-transmitted MMTV DNA and mammary tumorigenesis in several C3Hf sublines and C3H/StWi mice. Mammary tumorigenesis in C3Hf mice is strongly influenced by hormones. Several C3Hf sublines will be examined because C3H/Bi and C3H/St mice have different MMTV DNA sequences and because C3Hf/He and C3Hf/Ki tumors differ in their immunologic properties. Mammary tumorigenesis in C3H/StWi mice is not influenced by hormones but occurs following treatment with urethane or 7,12-dimethylbenz(a)anthracene. To examine the role of germinally-transmitted MMTV in mammary tumorigenesis in these mice, MMTV RNA expression will be quantitated in normal and neoplastic mammary tissues by molecular hybridization. The complexity of MMTV RNA will be examined by separation of MMTV RNA species on the basis of size and detection of MMTV RNAs by filter hybridization. The MMTV DNA sequences will be examined by restriction endonuclease mapping and correlated to MMTV expression. Recent studies have suggested that expression of specific genes is related to hypomethylation of the gene sequence. The relationship of methylation of MMTV DNA to MMTV RNA expression will be examined by digestion with isoschizomeric restriction endonucleases that differ in their ability to cleave methylated sequences. Through these studies a better understanding of the relationship of the germinally-transmitted MMTV to mammary tumorigenesis by hormones and chemical carcinogens will be obtained.